This invention relates to a keyboard structure that is adapted to be scan-controlled by an external digital controller or processor, such as a microprocessor. The keyboard might include a matrix of any form of analog keyswitches specifically illustrated in this detailed description as capacitive keyswitches activated individually by depression of manual keytops. A capacitive pad on each keyswitch overlays etched capacitive plates arranged in pairs on a printed circuit board. As the keytop is depressed, its pad capacitively couples the etched plates, thereby completing an electrical circuit including the paired plates.
Other forms of analog keyswitches might be utilized in the keyboard matrix, depending upon the details of a specific installation. Analog switches in general are electrical switches that produce a variable signal in the form of current, voltage or charge when actuated. The amplitude of the signal varies in general with switch travel. In the case of a capacitive switch it varies with the degree of separation between the movable capacitor element and the stationary capacitor elements. This is contrasted with mechanical switches, which are normally of a resistive contact type having a well defined open or closed state. The detection of analog switch actuation, using an economical arrangement of integrated circuits adapted to interface with controller or processor circuits, is the subject of this disclosure.
One object of the present design is to make efficient use of the processor circuits by enabling the entire keyboard matrix to be interrogated very rapidly. This requires simultaneous examination of more than one key, ruling out the known methods of using decoders and multiplexers in a synchronous "one key at a time" interrogation system. The chosen solution is a bus-oriented processor interface circuit in the form of integrated circuit chips. For maximum economy in relation to the usual size requirements of alphanumeric keyboards, two chips are utilized in the interface circuitry, each being packaged as a twenty pin part.
The present keyboard does not require a dedicated controller, although a controller can be incorporated on the keyboard if desired. The present system permits an external controller to be used for substantial functions other than keyboard operation, reducing total cost and equipment needs. It requires only a small fraction of the total operating capability of available microcomputer devices for keyboard-related functions in scanning and validating keyswitch use.
The resulting keyboard is extremely simple in both design and structure. The circuit board requires only the presentation of the etched plates and the usual conductors leading from them to the two integrated circuit chips. Ground line isolation is utilized in the matrix layout to keep the inactive keyswitch capacitance as low as possible, but ground line isolation is not required to the extent necessary in previous keyboards. It is also shown to be practical to design the keyboard in a manner that eliminates the requirement for external termination or biasing networks on the matrix lines. Such networks have become a common part of most capacitive switch designs and the keyboards utilizing them.